A projection-type image display device including a screen and a projection device which projects an image light beam on the screen has been widely used. In a typical projection-type image display device, a two-dimensional image as a original image is generated by using a spatial light modulator such as a liquid crystal micro display or a DMD (digital micromirror device), and the two-dimensional image is magnified and projected on a screen by using a projection optical system, so that an image is displayed on the screen.
As the projection device, various types including a commercialized product called an “optical type projector” have been proposed. In a general optical type projector, the spatial light modulator such as a liquid crystal display is illuminated by using an illumination device including a white light source such as a high pressure mercury lamp, and an obtained modulation image is magnified and projected on the screen by using lenses. For example, JP2004-264512A discloses a technique where a white light beam generated by a supper-high pressure mercury lamp is divided into three primary color components R, G, and B by a dichroic mirror, the light beams are guided to spatial light modulators corresponding to the primary colors, generated modulation images corresponding to the primary colors are combined by a cross dichroic prism to be projected on the screen.
However, a high intensity discharge lamp such as a high pressure mercury lamp has a relatively short lifecycle, and in the case where the lamp is used for an optical type projector or the like, the lamp needs to be frequently replaced. In addition, since a relatively large optical system such as a dichroic mirror is needed in order to extract the light beams of the primary color components, there is a problem in that the size of the whole device becomes large.
In order to cope with this problem, a type using a coherent light source such as a laser is also proposed. For example, a semiconductor laser which is widely used in industries has a very long lifecycle in comparison with the high intensity discharge lamp such as a high pressure mercury lamp. In addition, since the laser source is a light source capable of generating light having a single wavelength, a spectroscopic device such as a dichroic mirror is unnecessary, so that there is an advantage in that the whole device can be miniaturized.
On the other hand, in the type using the coherent light source such as a laser source, there is another problem in that speckles occur. The speckle is a punctate pattern which occurs when the coherent light beam such as a laser beam is irradiated on a scattering surface. If the speckle occurs on the screen, it is observed as punctate luminance unevenness (brightness unevenness), so that it becomes a factor of exerting physiological bad influence on the observer. The reason why the speckles occur in the case of using the coherent light beam is that the coherent light beams reflected from portions of the scattering reflection surface such as a screen have very high coherency, and the speckles are generated through interference therebetween. For example, in a literature “Speckle Phenomena in Optics, Joseph W. Goodman, Roberts & Co. 2006”, theoretical review of the occurrence of speckles is made in detail.
In the type of using the coherent light source, since there is an intrinsic problem in that the speckles occur, techniques for suppressing the occurrence of speckles have been proposed. For example, JP6-208089A discloses a technique where a scattering plate is irradiated with a laser beam, an obtained scattered light beam is guided to a spatial light modulator, and the scattering plate is driven to rotate by a motor, so that speckles are reduced.
With respect to the projection device and the projection-type image display device using the coherent light source, the techniques for reducing speckles, which have been proposed up to now as described above, may not effectively and sufficiently suppress the speckles. For example, in the method disclosed in JP6-208089A described above, since the laser beams irradiated on the scattering plate are scattered, a portion of the laser beams are lost without contribution to image display. In addition, although the scattering plate needs to be rotated in order to reduce the speckles, the mechanical rotation mechanism becomes a relatively large device, and the power consumption is increased. Moreover, although the scattering plate is rotated, the position of the optical axis of the illumination light beam is not changed, so that the speckles occurring on the screen due to the diffusion may not be sufficiently suppressed.
In addition, the problem of speckles is not a problem peculiar to only the projection device or the projection-type image display device, but the speckles cause a problem in various devices combined with an illumination device which illuminates an illuminated zone with a coherent light beam. For example, a scanner which reads out image information is combined with an illumination device which illuminates a read object. In the case where speckles occur due to a light beam which is illuminated on the read object, the image information may not accurately read. In order to prevent this problem, in a scanner using a coherent light beam, a special process such as image correction needs to be performed.
The coherent light beam generated from a single light source is typically a monochromatic light beam as tipified by a laser beam. In addition, the coherent light beam which is generated by a practically available light source is limited to a light beam having a specific wavelength (range). On the other hand, at present, in many cases, it is preferred that the illuminated zone is illuminated or an image is displayed in desired color, in plural colors, typically in full color which may not be displayed by the single light source. Therefore, at present, it is preferred that, in order to be adapted to various uses, the illumination device, the projection device, and projection-type display device which are actually developed be appropriately adapted to illuminating the illuminated zone with a plurality of coherent light beams having mutually different wavelength ranges or to displaying an image by using a plurality of coherent light beams having mutually different wavelength ranges.